In the case of an impact-- or shock-type supercharging, it is known to combine the individual engine cylinders of an internal-combustion engine, as a function of the firing sequence, in groups such that the pressure waves can form in an undisturbed manner. In the case of engines with a fairly large number of cylinders, several separate exhaust gas pipes and corresponding connected separate turbine flows of the exhaust gas turbochargers are therefore required so that the pressure impacts or shocks in the exhaust gas pipes can fully develop.
In Swiss Patent CH-PS 627 821, the corresponding exhaust gas pipes are arranged in a gas-tight water-cooled housing. The exhaust gas pipes consist of individual pipe sections which are fitted together by plug-type connections. The plug-type connections are constructed without seals. Exhaust gas which flows out at the leakage points is held back in the space formed by the housing and the exhaust gas pipes. After a corresponding pressure has built up in the space, a further flow-off of exhaust gas from the exhaust gas pipes is prevented.
German Patent DE 36 29 841 A1 also describes an exhaust system for the impact-- or shock-type supercharging of an internal-combustion engine. Three cylinders, respectively, of the internal-combustion engine are connected with an exhaust gas pipe which leads to separate turbines of two exhaust gas turbochargers. Upstream of the turbines, bypass pipes branch off the exhaust gas pipes and lead into a common blow-off valve. A common bypass pipe leads from the blow-off valve into a common pipe connected with the outlet connections of the turbines. The blow-off valve permits the leading of the exhaust gas past the turbines. The reason is that in order to achieve a good acceleration behavior of the exhaust gas turbocharger in the case of a partial load, the use of narrow turbine cross-sections is required. This has the result that, in the case of a high load, a portion of the exhaust gases must be led past the turbine in an unutilized manner so that, while the acceleration-optimized construction of the exhaust gas pipes and the exhaust gas turbochargers is maintained, good engine operating values are obtained also in the full-load point. The indicated blow-off valve makes it possible to extend the bypass pipes fluidically separately from one another so that it is ensured that the scavenging operations are not disturbed by pressure impacts or shocks of cylinders which are mowing out at the time.
Also in the internal-combustion engine according to Swiss Patent CH 188 402, the exhaust gases are supplied to a turbine in order to utilize their energy content. To compensate the pressure fluctuations in the exhaust gas pipe leading to the turbine, a compensating space is provided into which a portion of the exhaust gases flows off in the case of each exhaust stroke. However, no blow-off valve is provided. For discharging the impurities depositing from the exhaust gases, however, a valve is mounted which is constantly open during the operation. In addition, the compensation space is provided with a pipe and with a flap situated therein which, however, is opened only when the turbine is switched on and off and is, at any rate, not used for controlling the turbine.
An object of the present invention is to provide a simple device for blowing-off exhaust gas for a multi-walled exhaust system by way of which the impact- or shock-type supercharging is not impaired.
This object has been achieved in accordance with the present invention by providing a bypass pipe which connects a space with the outlet pipe, with a bypass valve arranged in the bypass pipe to control the exhaust gas quantity passing therethrough.
The blowing-off of exhaust gas takes place from the space which is formed between the exhaust-gas-carrying components and the housing. As a result of openings in the exhaust-gas-carrying components, approximately the medium pressure in the space builds up in front of the turbine. Because of the low pressure difference with respect to the pulsating pressure existing in the exhaust-gas-carrying components, the losses in the openings are minimal. That is, the impact-- or shock-type supercharging is not disturbed.
When the turbine output is to be reduced for limiting the charge pressure, the pressure in the space is lowered by the opening of the blow-off valve. As a result of the pressure difference, a blowing-off of exhaust gas is achieved. In the case of this exhaust system, the exhaust gas pipes leading to the turbines are arranged only inside the housing. However, only the turbines or the exhaust gas pipes leading to the turbines as well as the turbines can also be arranged in the housing.
It is advantageous to arrange the blow-off valve in the direct proximity of the outlet of the turbines in the housing, whereby long by-pass pipes are avoided. If the exhaust-gas-carrying components, particularly the exhaust gas pipes, are constructed with plug-type connections, exhaust gas may flow out from the exhaust gas pipes by way of leakage points into in the plug-type connections into the space.
It may also be advantageous to provide special bores which may be used as exhaust gas transfer points. The bores in the two exhaust gas pipes are to be arranged such that the impact-- or shock-type supercharging is not disturbed. This can take place by arranging the bores to face away from one another or they can be disposed far apart from one another.
It is a particular advantage for the bores to be situated in direct proximity of the outlet to the bypass pipe because then the exhaust gas quantity to be blown off will enter the bypass pipe directly without causing any strong flow movements in the more remote areas of the space. Strong flow movements are undesirable because, as a result more heat is transferred into the cooled housing and the energy content of the exhaust gas flow is therefore reduced.